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LT6600IDF-2.5#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
LT6600-2.5
7
660025fe
PIN FUNCTIONS
IN
–
and IN
+
(Pins 1, 12/Pins 1, 8): Input Pins. Signals can
be applied to either or both input pins through identical
external resistors, R
IN
. The DC gain from differential inputs
to the differential outputs is 1580/R
IN
.
NC (Pins 2, 5, 11/NA): No Connection
V
OCM
(Pin 3/Pin 2): DC Common Mode Reference Voltage-
for the 2nd Filter Stage. Its value programs the common
mode voltage of the differential output of the fi lter. This
is a high impedance input, which can be driven from an
external voltage reference, or it can be tied to V
MID
on the
PC board. V
OCM
should be bypassed with a 0.01F ceramic
capacitor unless it is connected to a ground plane.
V
+
and V
–
(Pins 4, 8, 9/Pins 3, 6): Power Supply Pins. For
a single 3.3V or 5V supply (V
–
grounded) a quality 0.1F
ceramic bypass capacitor is required from the positive
supply pin (V
+
) to the negative supply pin (V
–
). The bypass
should be as close as possible to the IC. For dual supply
applications, bypass V
+
to ground and V
–
to ground with
a quality 0.1F ceramic capacitor.
OUT
+
and OUT
–
(Pins 6, 7/Pins 4, 5): Output Pins. These
are the fi lter differential outputs. Each pin can drive a 100
and/or 50pF load to AC ground.
V
MID
(Pin 10/Pin 7): The V
MID
pin is internally biased at
mid-supply, see Block Diagram. For single supply op-
eration, the V
MID
pin should be bypassed with a quality
0.01F ceramic capacitor to V
–
. For dual supply operation,
V
MID
can be bypassed or connected to a high quality DC
ground. A ground plane should be used. A poor ground
will increase noise and distortion. V
MID
sets the output
common mode voltage of the 1st stage of the fi lter. It has
a 5.5k impedance, and it can be overridden with an
external low impedance voltage source.
(DFN/SO)
BLOCK DIAGRAM
–
+
–
+
V
OCM
–
–
+
+
V
OCM
1580Ω
1580Ω
800Ω
800Ω
800Ω
800Ω
V
+
V
–
11k
11k
OP AMP
PROPRIETARY
LOWPASS
FILTER STAGE
V
IN
–
V
IN
+
R
IN
R
IN
660025 BD
IN
+
V
OCM V
+
OUT
+
OUT
–
V
–
V
MID
IN
–
LT6600-2.5
8
660025fe
APPLICATIONS INFORMATION
Figure 1. (S8 Pin Numbers)
Interfacing to the LT6600-2.5
Note: The referenced pin numbers correspond to the S8
package. See the Pin Functions for the equivalent DFN-12
package pin numbers.
The LT6600-2.5 requires two equal external resistors, R
IN
,
to set the differential gain to 1580/R
IN
. The inputs to the
fi lter are the voltages V
IN
+
and V
IN
–
presented to the see
external components, Figure 1. The difference between
V
IN
+
and V
IN
–
is the differential input voltage. The aver-
age of V
IN
+
and V
IN
–
is the common mode input voltage.
Similarly, the voltages V
OUT
+
and V
OUT
–
appearing at Pins 4
and 5 of the LT6600-2.5 are the fi lter outputs. The differ-
ence between V
OUT
+
and V
OUT
–
is the differential output
voltage. The average of V
OUT
+
and V
OUT
–
is the common
mode output voltage.
Figure 1 illustrates the LT6600-2.5 operating with a single
3.3V supply and unity passband gain; the input signal is
DC-coupled. The common mode input voltage is 0.5V, and
the differential input voltage is 2V
P-P
. The common mode
output voltage is 1.65V, and the differential output voltage
is 2V
P-P
for frequencies below 2.5MHz. The common mode
output voltage is determined by the voltage at V
OCM
. Since
V
OCM
is shorted to V
MID
, the output common mode is the
mid-supply voltage. In addition, the common mode input
voltage can be equal to the mid-supply voltage of V
MID
.
Figure 2 shows how to AC couple signals into the LT6600-2.5.
In this instance, the input is a single-ended signal.
AC-coupling allows the processing of single-ended or
differential signals with arbitrary common mode levels.
The 0.1F coupling capacitor and the 1580 gain set-
ting resistor form a highpass fi lter, attenuating signals
below 1kHz. Larger values of coupling capacitors will
proportionally reduce this highpass 3dB frequency.
In Figure 3 the LT6600-2.5 is providing 12dB of gain. The
common mode output voltage is set to 2V.
Figure 2. (S8 Pin Numbers)
Figure 3. (S8 Pin Numbers)
–
+
1580Ω
1580Ω
0.01µF
0.1µF
3.3V
–
+
V
IN
–
V
IN
+
3
4
1
7
2
8
5
6
660025 F01
V
OUT
+
V
OUT
–
V
t
3
2
1
V
IN
+
V
IN
–
V
t
3
2
1
V
OUT
+
LT6600-2.5
V
OUT
–
0 0
–
+
1580
1580
0.01µF
0.1µF
0.1µF
0.1µF
3.3V
–
+
V
IN
+
3
4
1
7
2
8
5
6
660025 F02
V
OUT
+
V
OUT
–
V
3
2
1
t
0
LT6600-2.5
V
OUT
+
V
OUT
–
2
V
t
1
0
–1
V
IN
+
–
+
402Ω
402Ω
0.1µF
0.01µF
5V
–
+
V
IN
–
V
IN
+
3
4
1
7
2
8
5
6
660025 F03
V
OUT
+
V
OUT
–
+
–
2V
V
t
3
2
1
0
V
OUT
+
V
OUT
–
LT6600-2.5
V
t
3
2
1
0
V
IN
+
V
IN
–
500mV
P-P
(DIFF)
LT6600-2.5
9
660025fe
APPLICATIONS INFORMATION
Use Figure 4 to determine the interface between the
LT6600-2.5 and a current output DAC. The gain, or “trans-
impedance,” is defi ned as A = V
OUT
/I
IN
. To compute the
transimpedance, use the following equation:
A =
1580 •R1
R1+ R2
()
Ω
()
By setting R1 + R2 = 1580, the gain equation reduces
to A = R1().
The voltage at the pins of the DAC is determined by R1,
R2, the voltage on V
MID
and the DAC output current.
Consider Figure 4 with R1 = 49.9 and R2 = 1540. The
voltage at V
MID
, for V
S
= 3.3V, is 1.65V. The voltage at the
DAC pins is given by:
V
DAC
= V
PIN7
•
R1
R1+ R2 + 1580
+I
IN
•
R1• R2
R1+ R2
= 26mV +I
IN
• 48.3Ω
I
IN
is I
IN
+
or I
IN
–
. The transimpedance in this example is
49.6.
Evaluating the LT6600-2.5
The low impedance levels and high frequency operation
of the LT6600-2.5 require some attention to the matching
networks between the LT6600-2.5 and other devices. The
previous examples assume an ideal (0) source imped-
ance and a large (1k) load resistance. Among practical
examples where impedance must be considered is the
evaluation of the LT6600-2.5 with a network analyzer.
Figure 5 is a laboratory setup that can be used to charac-
terize the LT6600-2.5 using single-ended instruments
with 50 source impedance and 50 input impedance.
For a 12dB gain confi guration the LT6600-2.5 requires a
402 source resistance yet the network analyzer output is
calibrated for a 50 load resistance. The 1:1 transformer,
53.6 and 388 resistors satisfy the two constraints
above. The transformer converts the single-ended source
into a differential stimulus. Similarly, the output of the
LT6600-2.5 will have lower distortion with larger load
resistance yet the analyzer input is typically 50. The 4:1
turns (16:1 impedance) transformer and the two 402
resistors of Figure 5, present the output of the LT6600-2.5
with a 1600 differential load, or the equivalent of 800
to ground at each output. The impedance seen by the
network analyzer input is still 50, reducing refl ections in
the cabling between the transformer and analyzer input.
Differential and Common Mode Voltage Ranges
The rail-to-rail output stage of the LT6600-2.5 can process
large differential signal levels. On a 3V supply, the output
signal can be 5.1V
P-P
. Similarly, a 5V supply can support
signals as large as 8.8V
P-P
. To prevent excessive power
dissipation in the internal circuitry, the user must limit
differential signal levels to 9V
P-P
.
The two amplifi ers inside the LT6600-2.5 have indepen-
dent control of their output common mode voltage (see
the Block Diagram section). The following guidelines will
optimize the performance of the fi lter.
Figure 4. (S8 Pin Numbers) Figure 5. (S8 Pin Numbers)
–
+
0.1µF
3.3V
–
+
LT6600-2.5
3
4
1
0.01µF
CURRENT
OUTPUT
DAC
7
2
8
5
V
OUT
+
V
OUT
–
660025 F04
6
R2
R1
I
IN
–
I
IN
+
R2
R1
=
V
OUT
+
– V
OUT
–
I
IN
+
– I
IN
–
1580 • R1
R1 + R2
–
+
0.1µF
0.1µF
2.5V
–2.5V
–
+
LT6600-2.5
3
4
1
7
2
8
5
6
660025 F05
402Ω
402Ω
NETWORK
ANALYZER
INPUT
50Ω
COILCRAFT
TTWB-16A
4:1
NETWORK
ANALYZER
SOURCE
COILCRAFT
TTWB-1010
1:1
50Ω
53.6Ω
388Ω
388Ω
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16

LT6600IDF-2.5#PBF

Mfr. #:
Buy LT6600IDF-2.5#PBF
Manufacturer:
Analog Devices Inc.
Description:
Differential Amplifiers Very L N, Diff Amp & 2.5MHz Lpass Filt
Lifecycle:
New from this manufacturer.
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